1. Field of the Invention
The present invention generally relates to lithography, and more particularly to a method and system for mask inspection.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
To image smaller features, it has been proposed to use extreme ultraviolet radiation (EUV) with a wavelength in the range of 5-20 nanometers, in particular, 13.5 nanometers, or a charged particle beam, e.g., an ion beam and an electron beam, as the exposure radiation in a lithographic apparatus. These types of radiation need the beam path in the apparatus to be evacuated to avoid absorption. Since there are no known materials for making a refractive optical element for EUV radiation, EUV lithographic apparatus use mirrors in the radiation, illumination and projection systems. Such mirrors are highly susceptible to contamination, thereby reducing their reflectivity and hence the throughput of the apparatus. Further, sources for EUV may produce debris whose entry into the illumination system should be avoided.
As the dimensions of ICs decrease and the patterns being transferred from the mask to the substrate become more complex, detecting irregularities, defects, etc. (herein after defects) associated with a pattern formed on the mask becomes increasingly important. Consequently, defects in the features formed on the mask translate into pattern defects formed on the substrate. Mask defects can come from a variety of sources such as, for example, defects in coatings on mask blanks, the mask patterning process in a mask shop, and mask handling and contamination defects in a wafer fabrication facility. Therefore, inspection of masks for defects is important to minimize or remove unwanted particles and contaminants from affecting the transfer of a mask pattern onto the substrate.
Masks are inspected for any possible defects using pattern imaging and analysis systems. One way to detect defects is by comparing optical images from nominally identical patterns. Differences between the compared optical images can indicate defect areas. Another way to detect defects is by comparing the inspected pattern to a design database with differences indicating defect areas. However, pattern imaging and analysis systems tend to be slow, expensive, and resolution can be limited.
Laser scanning systems are used to inspect masks to detect presence of defects generated by contamination particles. The contamination is detected by detecting scattered light produced by the particles. These systems are used in particular for inspecting mask blanks or pellicles protecting mask patterns. However, the laser scanning systems are limited in their particle size resolution, particularly on EUV patterned masks. The pattern is etched into an absorber layer, which has a significant scattering cross-section. Scattered light produced by the pattern in the absorber layer can make it impossible to detect scattered light produced by a small particle.
Fourier filters have been proposed to block light scattered from an etched pattern and to pass light scattered from random defects. However, Fourier filters are pattern specific and must be tuned for each and every pattern. Programmable Fourier filters have been proposed, but only filtering the etched pattern in not efficient enough.